1. Field of the Invention
This invention relates generally to a method and apparatus for measuring a borehole and the surrounding earth formations and, more particularly, to a technique for calculating the diameter of a borehole and the formation resistivity from electromagnetic (xe2x80x9cEMxe2x80x9d) measurements obtained with a well tool. The invention has general application in the well logging art, but is particularly useful in measuring-while-drilling.
2. Description of Related Art
In order to improve oil and gas drilling and production operations, it is necessary to gather as much information as possible on the properties of the underground earth formation as well the environment in which drilling takes place. Such properties include characteristics of the earth formations traversed by a well borehole, in addition to data on the size and configuration of the borehole itself. Among the characteristics of the earth formation of interest to drillers and petrophysicists is the resistivity of the rock or strata surrounding the borehole. However, the processes often employed to measure these characteristics are subject to significant errors unless information on the borehole size and configuration is also taken into account in their determination. Knowledge of the borehole size is also useful to estimate the hole volume, which, in turn, is used to estimate the volume of cement needed for setting casing or when hole stability is of concern during drilling.
The collection of downhole information, also referred to as logging, is realized in different ways. A well tool, comprising transmitting and detecting devices for measuring various parameters, can be lowered into the borehole on the end of a cable, or wireline. The cable, which is attached to some sort of mobile processing center at the surface, is the means by which parameter data is sent up to the surface. With this type of wireline logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is being pulled uphole.
An improvement over wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance and avoid financial injury due to well damage such as collapse or fluid loss. Formation information collected during drilling also tends to be less affected by the mud fluid invasion processes or other undesirable influences as a result of borehole penetration, and therefore are closer to the properties of the virgin formation.
Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are know as logging-while-drilling (xe2x80x9cLWDxe2x80x9d). However, the terms MWD and LWD are often used interchangeably, and the use of either term in the present disclosure should be understood to include both the collection of formation and borehole information, as well as data on movement of the drilling assembly.
It is known in the art to measure the diameter, also known as the caliper, of a borehole to correct formation measurements that are sensitive to size or standoff. These corrections are necessary for accurate formation evaluation. U.S. Pat. No. 4,407,157 describes a technique for measuring a borehole caliper by incorporating a mechanical apparatus with extending contact arms that are forced against the sidewall of the borehole. This technique has practical limitations. In order to insert the apparatus in the borehole, the drillstring must be removed, resulting in additional cost and downtime for the driller. Such mechanical apparatus are also limited in the range of diameter measurement they provide.
Due to the unsuitability of mechanical calipers to drilling operations, indirect techniques of determining borehole calipers have been proposed. Conventional caliper measurement techniques include acoustic transducers that transmit ultrasonic signals to the borehole wall. U.S. Pat. No. 5,469,736 describes an apparatus for measuring the caliper of a borehole by transmitting ultrasonic signals during a drilling operation. U.S. Pat. No. 5,397,893 describes a method for analyzing formation data from a MWD tool incorporating an acoustic caliper. U.S. Pat. No. 5,886,303 describes a logging tool including an acoustic transmitter for obtaining the borehole caliper while drilling. However, the techniques proposed with acoustic calipers entail measurements employing standoff and travel time calculations, resulting in data of limited accuracy. Sound wave reflections in soft formations may also be too weak to be accurately detected, leading to loss of signals.
U.S. Pat. No. 4,899,112 (assigned to the assignee of the present invention) discloses a technique for determining a borehole caliper and formation resistivities from downhole measurements obtained with a well tool. This technique determines the borehole caliper and formation resistivity by computing phase differences and attenuation levels from measured data. U.S. Pat. No. 5,900,733 (assigned to the assignee of the present invention) discloses a technique for determining borehole fluid resistivities and diameters by examining the phase shift, phase average, and attenuation of signals from multiple transmitter and receiver locations via electromagnetic wave propagation. However, the disclosed technique is limited to a well tool of wave propagation type incorporating multiple transmitter means with particular spacing conditions. The disclosed technique is also limited in application to boreholes of limited diameters.
An emerging application of borehole caliper measurements, which is not geared towards formation evaluation, is the monitoring of borehole stability while drilling. Keeping track of the dynamic development of the borehole is critical for the successful completion of a well when borehole stability is of issue. In some instances, the borehole has been severely washed out with diameters in excess of seventy inches. This requires a borehole caliper measurement technique that works for a wide range of hole sizes. Not only should the caliper measurement indicate a large hole when severe washout occurs, it should also provide an accurate estimate of the required cement volume for casing upon completion of the well.
It is desirable to obtain a simplified method and system for accurately determining a borehole caliper and the surrounding formation resistivity utilizing EM measurements from a well tool. Still further, it is desired to implement a borehole caliper and formation resistivity calculation technique that works for a wide range of borehole sizes.
A method and system are provided for determining a caliper of a borehole and/or the resistivity of the surrounding earth formations utilizing a well tool disposed in the borehole. In one aspect of the invention, a process for determining the caliper of a borehole comprises modeling responses of the well tool at different borehole and formation parameters; determining a known borehole fluid parameter; transmitting electromagnetic energy into the borehole from a station on the tool; receiving electromagnetic energy at arbitrary multiple sensor stations on the tool; generating signal data, associated with the received energy, from the tool; recording downhole and/or transmitting the signal uphole; and matching the signal data and the known borehole fluid parameter against the modeled responses to determine the borehole caliper.
In another aspect of the invention, a process for determining the resistivity of an earth formation is provided. This process is similar to the process provided for determining a caliper of a borehole, with minor variations.
In another aspect of the invention, a borehole caliper/formation resistivity calculation system, including a multi-sensor well tool and an apparatus, is provided. The apparatus is adapted to be coupled to the well tool, the tool being disposed in a borehole in an earth formation and adapted to transmit EM energy into the formation from a tool station and to receive EM energy at arbitrary multiple tool sensor stations. The apparatus is further adapted to respond to signal data, generated by the tool and associated with the received energy, for determining the borehole caliper and/or formation resistivity. The apparatus comprises means for receiving the signal data; means for receiving a known borehole fluid parameter; means for generating and storing modeled well tool response data; and means for processing the signal data, the known fluid parameter, and the modeled response data to determine the borehole caliper and/or formation resistivity.